Many designs have been developed for snap closures for containers. Typical of such designs are U.S. Pat. Nos. 3,805,994; 4,561,562; 4,209,107; 4,574,974; 4,782,976; 5,373,955; 5,383,565; 4,256,240; 3,320,993; 3,101,864; 4,166,548; 2,711,840; 3,339,786; 3,809,284; design 243,231; and design 374,822.
Lacking in the prior designs is a simple structure for a snap fit lid which firmly engages the container internally, as well as at its edge, to form a double seal while additionally providing the benefits of thermal insulation to keep contents warm or cold, and incorporating into the design a venting feature which operates bidirectionally to relieve internal overpressure or vacuum due to temperature effects from the contents in the container. Apart from the simple construction incorporating a double seal, the physical execution of the double seal enhances the grip of the closure on the container by preferentially applying as much of the available forces accumulated from assembling the snap fit lid to the container into contact forces to maintain the internal and external seals. The aspects of the design of the present invention referred to above will be more clearly understood from a review of the preferred embodiment described below.